程序实现对输入时钟信号的7分频，程序采用两个计数器，一个由输入时钟的上升沿触发，另一个由时钟的下降沿触发，最后将2个计数器的输出相或，即得到占空比为50 的方波。-Program realizes frequency devision-by-7 of the input clock signal , the program uses two counters, one triggered by the rising edge of the input clock, and the other triggered by the falling edge of the clock.Then I operate "or" arithmetic to the output of two counters.In the last,I obtained the square wave of 50　duty ratio.